CH. XV] DEMONSTRATING REFRACTIVE EYE DEFECTS 671 



same time. If, however, direct current is available the demon- 

 stration is successful with one projector. 

 There are needed : 



(1) A trial frame with half the lens of 4 diopters and half of 

 3 diopters (fig. 400). 



(2) A trial frame with half the lens of i diopter and the other 

 half of plane glass. 



(3) A lantern slide of fig. 401. 



For the demonstration the arc lamp and the lens should be so 

 related that the image of the source of light is rather large as shown 



Left Right 



Eye Eye 



FIG. 401. LANTERN SLIDE FOR THE 

 FIG. 400. DOUBLE TRIAL DEMONSTRATION OF UNLIKE REFRAC- 



LENSES FOR UNLIKE RE- TION IN THE Two EYES - 



FRACTION IN THE TWO 

 EYES. 



by the concentric circles in fig. 400. The light must be accurately 

 centered also. 



Put the lens in the metal holder and the special lantern slide in 

 its carrier and move the slide up to a point 27 to 28 centimeters 

 from the lens. The image of the right eye (4 diopter lens) will be 

 sharp, and that of the left eye will be blurred. Now pull the slide 

 back to a distance of 36-37 centimeters from the lens and the left 

 eye image will be sharp and the right eye blurred. This is com- 

 parable to a defect of myopia in one eye and hyperopia in the 

 other one eye is short sighted and one long sighted. Put the 

 slide back in position for the 4 diopter lens so that the right eye 

 will be in focus. Now put in front of the lens the correcting lens 

 of i diopter for the left half. This will make both sides of the lens 

 4 diopters and both images will be sharp as in normal vision (fig. 

 402, A.B.). 



It will be seen that the blurred, left-eye image (fig. 402 A) is 

 smaller than the sharp right-eye image. This is because the 3 



